[Clang][AArch64] Fix Pure Scalables Types argument passing and return (#112747)

Pure Scalable Types are defined in AAPCS64 here:

https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#pure-scalable-types-psts

And should be passed according to Rule C.7 here:

https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#682parameter-passing-rules

This part of the ABI is completely unimplemented in Clang, instead it
treats PSTs sometimes as HFAs/HVAs, sometime as general composite types.

This patch implements the rules for passing PSTs by employing the
`CoerceAndExpand` method and extending it to:
* allow array types in the `coerceToType`; Now only `[N x i8]` are
considered padding.
* allow mismatch between the elements of the `coerceToType` and the
elements of the `unpaddedCoerceToType`; AArch64 uses this to map
fixed-length vector types to SVE vector types.

Corectly passing a PST argument needs a decision in Clang about whether
to pass it in memory or registers or, equivalently, whether to use the
`Indirect` or `Expand/CoerceAndExpand` method. It was considered
relatively harder (or not practically possible) to make that decision in
the AArch64 backend.
Hence this patch implements the register counting from AAPCS64 (cf.
`NSRN`, `NPRN`) to guide the Clang's decision.

Author: momchil-velikov

clang/include/clang/CodeGen/CGFunctionInfo.h

@@ -271,12 +271,8 @@ class ABIArgInfo {
     // in the unpadded type.
     unsigned unpaddedIndex = 0;
     for (auto eltType : coerceToType->elements()) {
-      if (isPaddingForCoerceAndExpand(eltType)) continue;
-      if (unpaddedStruct) {
-        assert(unpaddedStruct->getElementType(unpaddedIndex) == eltType);
-      } else {
-        assert(unpaddedIndex == 0 && unpaddedCoerceToType == eltType);
-      }
+      if (isPaddingForCoerceAndExpand(eltType))
+        continue;
       unpaddedIndex++;
     }
 
@@ -295,12 +291,8 @@ class ABIArgInfo {
   }
 
   static bool isPaddingForCoerceAndExpand(llvm::Type *eltType) {
-    if (eltType->isArrayTy()) {
-      assert(eltType->getArrayElementType()->isIntegerTy(8));
-      return true;
-    } else {
-      return false;
-    }
+    return eltType->isArrayTy() &&
+           eltType->getArrayElementType()->isIntegerTy(8);
   }
 
   Kind getKind() const { return TheKind; }

clang/lib/CodeGen/CGCall.cpp

@@ -1410,6 +1410,30 @@ static Address emitAddressAtOffset(CodeGenFunction &CGF, Address addr,
   return addr;
 }
 
+static std::pair<llvm::Value *, bool>
+CoerceScalableToFixed(CodeGenFunction &CGF, llvm::FixedVectorType *ToTy,
+                      llvm::ScalableVectorType *FromTy, llvm::Value *V,
+                      StringRef Name = "") {
+  // If we are casting a scalable i1 predicate vector to a fixed i8
+  // vector, first bitcast the source.
+  if (FromTy->getElementType()->isIntegerTy(1) &&
+      FromTy->getElementCount().isKnownMultipleOf(8) &&
+      ToTy->getElementType() == CGF.Builder.getInt8Ty()) {
+    FromTy = llvm::ScalableVectorType::get(
+        ToTy->getElementType(),
+        FromTy->getElementCount().getKnownMinValue() / 8);
+    V = CGF.Builder.CreateBitCast(V, FromTy);
+  }
+  if (FromTy->getElementType() == ToTy->getElementType()) {
+    llvm::Value *Zero = llvm::Constant::getNullValue(CGF.CGM.Int64Ty);
+
+    V->setName(Name + ".coerce");
+    V = CGF.Builder.CreateExtractVector(ToTy, V, Zero, "cast.fixed");
+    return {V, true};
+  }
+  return {V, false};
+}
+
 namespace {
 
 /// Encapsulates information about the way function arguments from
@@ -3196,26 +3220,14 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
       // a VLAT at the function boundary and the types match up, use
       // llvm.vector.extract to convert back to the original VLST.
       if (auto *VecTyTo = dyn_cast<llvm::FixedVectorType>(ConvertType(Ty))) {
-        llvm::Value *Coerced = Fn->getArg(FirstIRArg);
+        llvm::Value *ArgVal = Fn->getArg(FirstIRArg);
         if (auto *VecTyFrom =
-                dyn_cast<llvm::ScalableVectorType>(Coerced->getType())) {
-          // If we are casting a scalable i1 predicate vector to a fixed i8
-          // vector, bitcast the source and use a vector extract.
-          if (VecTyFrom->getElementType()->isIntegerTy(1) &&
-              VecTyFrom->getElementCount().isKnownMultipleOf(8) &&
-              VecTyTo->getElementType() == Builder.getInt8Ty()) {
-            VecTyFrom = llvm::ScalableVectorType::get(
-                VecTyTo->getElementType(),
-                VecTyFrom->getElementCount().getKnownMinValue() / 8);
-            Coerced = Builder.CreateBitCast(Coerced, VecTyFrom);
-          }
-          if (VecTyFrom->getElementType() == VecTyTo->getElementType()) {
-            llvm::Value *Zero = llvm::Constant::getNullValue(CGM.Int64Ty);
-
+                dyn_cast<llvm::ScalableVectorType>(ArgVal->getType())) {
+          auto [Coerced, Extracted] = CoerceScalableToFixed(
+              *this, VecTyTo, VecTyFrom, ArgVal, Arg->getName());
+          if (Extracted) {
             assert(NumIRArgs == 1);
-            Coerced->setName(Arg->getName() + ".coerce");
-            ArgVals.push_back(ParamValue::forDirect(Builder.CreateExtractVector(
-                VecTyTo, Coerced, Zero, "cast.fixed")));
+            ArgVals.push_back(ParamValue::forDirect(Coerced));
             break;
           }
         }
@@ -3326,16 +3338,33 @@ void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
       ArgVals.push_back(ParamValue::forIndirect(alloca));
 
       auto coercionType = ArgI.getCoerceAndExpandType();
+      auto unpaddedCoercionType = ArgI.getUnpaddedCoerceAndExpandType();
+      auto *unpaddedStruct = dyn_cast<llvm::StructType>(unpaddedCoercionType);
+
       alloca = alloca.withElementType(coercionType);
 
       unsigned argIndex = FirstIRArg;
+      unsigned unpaddedIndex = 0;
       for (unsigned i = 0, e = coercionType->getNumElements(); i != e; ++i) {
         llvm::Type *eltType = coercionType->getElementType(i);
         if (ABIArgInfo::isPaddingForCoerceAndExpand(eltType))
           continue;
 
         auto eltAddr = Builder.CreateStructGEP(alloca, i);
-        auto elt = Fn->getArg(argIndex++);
+        llvm::Value *elt = Fn->getArg(argIndex++);
+
+        auto paramType = unpaddedStruct
+                             ? unpaddedStruct->getElementType(unpaddedIndex++)
+                             : unpaddedCoercionType;
+
+        if (auto *VecTyTo = dyn_cast<llvm::FixedVectorType>(eltType)) {
+          if (auto *VecTyFrom = dyn_cast<llvm::ScalableVectorType>(paramType)) {
+            bool Extracted;
+            std::tie(elt, Extracted) = CoerceScalableToFixed(
+                *this, VecTyTo, VecTyFrom, elt, elt->getName());
+            assert(Extracted && "Unexpected scalable to fixed vector coercion");
+          }
+        }
         Builder.CreateStore(elt, eltAddr);
       }
       assert(argIndex == FirstIRArg + NumIRArgs);
@@ -3930,17 +3959,24 @@ void CodeGenFunction::EmitFunctionEpilog(const CGFunctionInfo &FI,
 
   case ABIArgInfo::CoerceAndExpand: {
     auto coercionType = RetAI.getCoerceAndExpandType();
+    auto unpaddedCoercionType = RetAI.getUnpaddedCoerceAndExpandType();
+    auto *unpaddedStruct = dyn_cast<llvm::StructType>(unpaddedCoercionType);
 
     // Load all of the coerced elements out into results.
     llvm::SmallVector<llvm::Value*, 4> results;
     Address addr = ReturnValue.withElementType(coercionType);
+    unsigned unpaddedIndex = 0;
     for (unsigned i = 0, e = coercionType->getNumElements(); i != e; ++i) {
       auto coercedEltType = coercionType->getElementType(i);
       if (ABIArgInfo::isPaddingForCoerceAndExpand(coercedEltType))
         continue;
 
       auto eltAddr = Builder.CreateStructGEP(addr, i);
-      auto elt = Builder.CreateLoad(eltAddr);
+      llvm::Value *elt = CreateCoercedLoad(
+          eltAddr,
+          unpaddedStruct ? unpaddedStruct->getElementType(unpaddedIndex++)
+                         : unpaddedCoercionType,
+          *this);
       results.push_back(elt);
     }
 
@@ -5472,6 +5508,8 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
     case ABIArgInfo::CoerceAndExpand: {
       auto coercionType = ArgInfo.getCoerceAndExpandType();
       auto layout = CGM.getDataLayout().getStructLayout(coercionType);
+      auto unpaddedCoercionType = ArgInfo.getUnpaddedCoerceAndExpandType();
+      auto *unpaddedStruct = dyn_cast<llvm::StructType>(unpaddedCoercionType);
 
       llvm::Value *tempSize = nullptr;
       Address addr = Address::invalid();
@@ -5502,11 +5540,16 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
       addr = addr.withElementType(coercionType);
 
       unsigned IRArgPos = FirstIRArg;
+      unsigned unpaddedIndex = 0;
       for (unsigned i = 0, e = coercionType->getNumElements(); i != e; ++i) {
         llvm::Type *eltType = coercionType->getElementType(i);
         if (ABIArgInfo::isPaddingForCoerceAndExpand(eltType)) continue;
         Address eltAddr = Builder.CreateStructGEP(addr, i);
-        llvm::Value *elt = Builder.CreateLoad(eltAddr);
+        llvm::Value *elt = CreateCoercedLoad(
+            eltAddr,
+            unpaddedStruct ? unpaddedStruct->getElementType(unpaddedIndex++)
+                           : unpaddedCoercionType,
+            *this);
         if (ArgHasMaybeUndefAttr)
           elt = Builder.CreateFreeze(elt);
         IRCallArgs[IRArgPos++] = elt;